In marine propulsion units having a gear case below the water line and at least one propeller mounted on a propeller shaft, at least one first seal is normally applied on the propeller shaft for preventing water from entering the drive housing and at least one second seal is normally applied on the propeller shaft for preventing transmission lubricants or oil from leaking out into the water.
Fishing lines, nets, weeds, and other debris can catch on the propeller, and can wind around the propeller shaft as it rotates. The fishing line, etc. can work into the housing of the propulsion unit and can damage the seals, thereby resulting in leakage of water into the housing and/or leakage of lubricants or oil out of the housing.
A common solution proposed for the entanglement issue in single propeller marine propulsion units is the provision of a line cutter, which is typically placed near the strut and operates by cutting fishing lines and debris away from the propeller shaft. However, such line cutters are high maintenance because placement near the strut makes installation and servicing difficult, in addition to exposing a potentially dangerous sharp cutting edge. Furthermore, line cutters do not exist for single or dual propeller systems with a gear case below the water line, which form a significant proportion of all marine propulsion units.